I was going to say, "holy crap, black females and single mexicans are stingy tippers even in South America??"just to agree with that.
worst tippers = tie between females (specially black ones) and single mexicans.
good tippers = middle age single white guys.
best tippers = gay male couple.
so, from my summer as a waiter while i was in college in the US, i can distinctly remember.... the worst possible table is a table with 4 single black females. you´ll be lucky to get $1 from each.
while the hispanics would leave a similar tip, they would much, much less demanding.